This invention relates to a high inlet artery which can be used with a thermosyphon in order to alleviate problems of thermosyphon flooding and its consequences.
A thermosyphon is a closed end tube with evaporator and condenser sections, which contains a working fluid in equilibrium between its liquid and vapor phases. When sufficient heat is applied to the bottom of the thermosyphon, the pool of liquid at the bottom of the thermosyphon begins to boil. Cooling the top end of the thermosyphon causes the vapor produced from the boiling liquid to condense on the walls of the condenser and, driven by the force of gravity, to drain back to the liquid pool at the bottom. Due to the fact that the working fluid is constantly close to its saturation temperature, the thermosyphon is very effective in transferring large amounts of heat across a small cross-sectional area with only a small drop in temperature.
Thermosyphons powered by gas burners have been successfully tested in home and industrial applications such as space heating. The thermosyphons proposed for these applications may include a series of finned tubes that are attached to manifolds at their tops and their bottoms. The tubes are evacuated, and then prior to their being sealed are charged with a working fluid such as water. In use, the tubes are placed with their evaporator section in one chamber receiving combustion products of a burner. In that chamber, hot combustion gases are blown over the evaporator section of the tubes. In another chamber, room air to be heated is blown over the condenser section of the tubes to remove heat from the condensing working fluid.
A problem with this method of heating has been that in some installations, the evaporator section of the thermosyphons has been known to overheat, causing the thermosyphon tubing to melt. This can occur when the working fluid evaporates more rapidly than it can be replenished or the liquid return to the evaporator is impeded by upward flowing vapor. This phenomenon is known as flooding.
Other problems associated with thermosyphons involve various limiting factors of the operation of the units. One such factor that affects the power output of a thermosyphon is the amount of working fluid in it. In general, an increase in the amount of working fluid leads to a higher operating limit. One reason for this is that a large fill charge increases the average liquid level and thus puts a greater supply of liquid into the evaporator which is likely to increase heat transfer and operating limits. As a result, in most space heating applications thermosyphon tubes are charged to the point where their evaporators are hydrostatically full of liquid. A disadvantage in using a large fill charge, however, is that such a charge in the evaporator section of a thermosyphon increases the temperature gradient of the working fluid thus decreasing heat transfer. Also, a large fill charge can result in more liquid remaining in the condenser section, which impedes condensation.
Another problem associated with the manifolded thermosyphon design is that if there is overheating in one section of the thermosyphon tubes, due to the tubes being in communication with one another, the entire unit will overheat and fail. To avoid this, the tubes can be separated so that if one of the tubes fails for any reason, it will not cause the entire unit to fail. However, separating the tubes so that each tube acts independently leads to a further and unacceptable decrease in the operating limit.
These problems have been dealt with to some degree by presently employed internal arteries, which are placed inside of thermosyphons to assist in downward transport of condensate. These arteries are positioned coaxially with the thermosyphon tube with their inlets adjacent to the thermosyphon tube wall at the bottom of the condenser section of the thermosyphon tube. As a result, some of the condensate, after it has traveled through the condenser section of the thermosyphon tube, is taken out of the path of the upwardly flowing vapor by flowing into and down through the artery. The arteries also have the effect of allowing the condensation to reach the bottom of the evaporator section of the thermosyphon more quickly than had the condensation traveled the length of the thermosyphon along the side wall against the resistance of the upward flowing vapor.
However, these known arteries do not alleviate the problem of flooding caused by the upwardly moving vapor interfering with the return of liquid. The vapor velocities range from zero at either end of the thermosyphon to their maximum value in the adiabatic transition section between the evaporator and condenser. An artery whose inlet is at the bottom of the condenser is, therefore, in a region of maximum vapor velocity. As a result, at and directly above the artery inlet the liquid return is impeded by the high vapor velocity in this region of the thermosyphon tube. Condensate must reach the bottom of the condenser before any benefit of the artery is possible.
As power throughput into a thermosyphon is increased, the average liquid level in the thermosyphon rises due to the increased vapor velocity. If the liquid level rises past the top of the known artery, it can impede the entrance of liquid into it. Since the known artery has its inlet at the bottom of the condenser, it is necessary to pick a fill that will keep the liquid level below the artery inlet. This can allow the average liquid level to drop below the top of the evaporator and possibly lower the operating limit.
Another disadvantage of known arteries is that they require tilting the thermosyphon to allow the condensate to collect and drain into the entrance of the artery. As a result, thermosyphon tubes using known arteries cannot be operated vertically.
As a result, there is a need for a means by which flooding conditions in a thermosyphon can be relieved so that thermosyphons can be operated under high power conditions that would otherwise cause evaporator overheating and failure of the device. There is also a need for a means by which the fill charge used in a thermosyphon can be increased to prevent the possibility of lack of liquid in the evaporator without the associated increase in temperature gradient and loss of condenser effectiveness.
It is therefore an object of the present invention to provide a means by which the efficiency with which the working fluid in a thermosyphon is evaporated and condensed is optimized.
It is another object of the present invention to circumvent flooding so that a thermosyphon can be operated under higher power conditions than has heretofore been possible.
It is yet another object of the present invention to prevent evaporator dry-out and thermosyphon overheating.
It is still another object of the present invention to allow a larger fill charge of working fluid to be used in a thermosyphon, thus preventing the average liquid level from dropping below the top of the evaporator and decreasing the operating limit.
These and other objects of the invention will be shown with reference to the following description of the invention and the figures, in which like reference numbers refer to like members throughout the various views.